1. Field of the Invention
This invention relates to a water phantom type dose distribution determining apparatus, particularly to a water phantom type dose determining apparatus allowing a rapid and accurate determination of the dose distribution in water which is suitably used for predicting in advance the dose distribution of radiation beam, before radiation therapy dependent on the use of a proton radiation therapy device or the like is undertaken.
2. Description of the Prior Art
Conventional cancer therapy based on radiation of active rays uses X-rays, gamma rays, electron beams, fast neutron beams, etc. These active rays, as shown in FIG. 6, become the strongest at sites close to the surface of a patient, and thus may inflict damages on normal tissues close to the body surface when those rays are directed towards a cancer in a deeper part of the body. By the way, a proton or a particle which comes into being when a hydrogen atom has been removed of the electron, has a positive charge, and has a mass 1836 times as large as that of electron, can be accelerated under a high energy state by an accelerator to give a proton beam. The proton beam is characterized by having the maximum dose peak or a Bragg peak P at a certain depth from the body surface, and then declining rapidly to zero.
This is because, as the electric force a proton A exerts on electrons becomes large in proportion to its proximity to the latter, when the proton has a high kinetic energy and runs at a high speed, the time for the proton to interact with nearby electrons is short, and ionization is small in magnitude, but, when it loses the kinetic energy to nearly make a stop, the time for interaction becomes long and ionization rapidly increases in magnitude.
Thanks to this nature peculiar to protons, it is possible to apply proton beams for cancer therapy keeping normal cells other than a cancer comparatively free from damages, even if the cancer lies in a deeper part of the body. Further, as the radiation-based biological effect (RBE) of a proton beam is nearly equal to that of x-rays, the proton radiation therapy is advantageous in that it can make the most of knowledge and experience accumulated in the field of conventional X-ray radiation therapy. With these features, the proton radiation therapy device is being introduced as a therapy means to treat a cancer without removing any functional organs and encroaching on the quality of life.
In the radiation therapy of cancer, it is ideal to concentrate a lethal dose of active rays onto the cancer alone without inflicting any irreversible damages to nearby normal tissues. The Proton radiation therapy, as shown in FIG. 6, exploits the feature characteristic with protons that a proton beam incident on a substance gives the maximum dose or Bragg peak P just before it ceases to move. Namely the therapy in question aims at achieving this ideal by covering only the cancerous lesion with that Bragg peak.
By the way, protons obtained from an accelerator are in the form of a slender beam, and its energy is constant (the depth of Bragg peak is also constant). On the other hand, cancerous lesions are varied in size and have complex shapes, and their depths in the body are not constant. Further, the density of tissues through which a proton beam must pass is not constant neither. Accordingly, to achieve an effective radiation therapy, it is necessary to (1) enlarge the proton beam to have a sufficient width to cover the whole cancer lesion in one radiation; (2) adjust the beam energy according to the depth of lesion; (3) give a sufficient energy distribution in depth so that the whole cancer lesion having a certain depth can receive a uniform irradiation; and (4) make corrections according to the irregularities in contour of the lesion, and in density of the tissues through which the proton beam must pass. To meet these requirements, a device as shown in FIG. 7 is introduced whereby an irradiation field is formed in accordance with the shape of a lesion to be radiated. To put it more specifically, a slender proton beam 20 transmitted to an irradiating section is passed through a scattering body 22 made of lead with a thickness of several millimeters to be converted into a wide beam 24 extending crosswise. Out of the wide beam 24 which widens in a conical form with the summit at the scattering body 22, picked up by a collimator described below is a portion which is close to the central axis and comparatively uniform in dose distribution. This beam gives an irradiation field of about ten and several centimeters in diameter necessary for therapy on a therapeutic platform below (not illustrated here). The widened beam 24 is incident on a fine degrader 26 which adjusts the maximum attainable depth in accordance with the depth of a lesion to be treated (for example, a tumor 12 in the patient's body 10). The fine degrader 26 is composed, for example, of two wedge-shaped acryl blocks 26a and 26b placed opposite to each other, and adjustment of overlaps of the two blocks 26a and 26b enables a continuous alteration of the thickness through which the proton beam must pass. The proton beam loses energy in accordance with the thickness through which it must pass, and thus the depth it can reach varies in accordance therewith. Thus, adjustment by means of this fine degrader 26 makes it possible for Bragg peak P shown in FIG. 6 to fall at the same depth at which the lesion requiring therapy lies.
The proton beam, after having passed the fine degrader 26, is incident on a ridge filter 28 which is introduced to confer an energy depth distribution AP to the proton beam in accordance with the thickness of tumor 12. The ridge filter 28 consists of metal rods placed in parallel like a series of steps which have different thickness with each other. Proton beams passing through the metal rods different in thickness have Bragg peaks P at different depths. Thus, expansion of the range of peaks or AP can be achieved by adjusting the width and height of those "steps" to give appropriate overlaps.
The proton beam, after having passed through the ridge filter 28, is incident on a block collimator 30 which roughly adjusts the planar form of proton beam. The reason why the block collimator 30 is introduced here for the adjustment of beam shape, in addition to a final collimator described later, is to prevent secondary radiation due to the block collimator from occurring close to the patient's body.
The proton beam, after having passed through the block collimator 30, is incident on a bolus 32 or a resin-made irregularly formed filter, for example, and receives corrections in accordance with the cross-sectional shape of tumor 12 at the maximum depth, and the irregularities of involved tissues. The shape of bolus is determined on the basis of the electron densities of nearby tissues determined from the contour line of tumor 12 and, for example, X-ray CT data of that tumor.
The proton beam, after having passed through the bolus 32, is incident on a final collimator 34 made of brass, for example, receives a final correction in accordance with the contour of planar shape of the tumor 12, and strikes the patient 10 as a therapeutic proton beam 36.
Prior to treatment, firstly, to check that the irradiation field is formed as initially designed, it is necessary to predict the actual dose distribution using a water phantom type dose distribution determining apparatus as shown in FIG. 8, which includes a water tank 42 equipped with a sensor 46 and filled with water 44 to simulate the absorption of active rays by the human body.
The conventional proton radiation therapy device is based on a horizontal or vertical static radiation, and, as the entire device including the irradiating section 120 is fixed rigidly, positioning of the device is determined manually each time experiment is undertaken: the water tank 42 has its top opened, and is placed on the treatment bed, or is placed on a wheeled cart 48.
With the conventional device, however, it is cumbersome to make a proper positioning, and properly maintain the level of water 44, and further water may spill while the water tank is being carried, to cause the water level to change. When the radiation therapy device is operated for actual treatments, it will be probably used at a frequency of once for every 20 minutes. Thus, checking the dose distribution by means of a water phantom type dose distribution determining apparatus must proceed rapidly. Further, as the device is handled by a physician or a radiological technician instead of an engineer, the apparatus must not require special techniques for its operation and must be easily manipulated. With the irradiating section 120 as shown in FIG. 1 (not publicly known) the inventors are designing in order to irradiate a properly shaped proton beam, the irradiating section 120 is mounted on a rotary irradiation chamber (to be referred to as gantry) rotatable round a therapeutic bed 200 on which a patient is fixed, and the irradiating section rotates 360.degree. round the patient during use. Thus, as shown by the broken line of FIG. 8, the incident angle .theta. of proton beam with respect to the surface of water 44 in the water tank 42 varies during rotation, but the device must be so constructed as not to allow those altered angles to affect the measurement results of dose distribution.